fix LRSC issue (RocketChip issue #86)

It was possible that the result of a store-conditional could get lost if it
did not depend on the result of the corresponding load-reserved.

This was because the MSHR does not update the client state based on the
secondary requests. So the LR would acquire the line in clientExcusiveClean,
but then we would fail to update the metadata array to change the state
to clientExclusiveDirty.

The solution is to track whether a secondary acquire would cause the
line to be dirty. If so, use M_XWR instead of the primary command to
generate the update coherence state.

rocket/src/main/scala/nbdcache.scala

@@ -282,6 +282,12 @@ class MSHR(id: Int)(implicit p: Parameters) extends L1HellaCacheModule()(p) {
   val s_invalid :: s_wb_req :: s_wb_resp :: s_meta_clear :: s_refill_req :: s_refill_resp :: s_meta_write_req :: s_meta_write_resp :: s_drain_rpq :: Nil = Enum(UInt(), 9)
   val state = Reg(init=s_invalid)
 
+  def stateIsOneOf(check_states: Seq[UInt]): Bool =
+    check_states.map(state === _).reduce(_ || _)
+
+  def stateIsOneOf(st1: UInt, st2: UInt*): Bool =
+    stateIsOneOf(st1 +: st2)
+
   val new_coh_state = Reg(init=ClientMetadata.onReset)
   val req = Reg(new MSHRReqInternal())
   val req_idx = req.addr(untagBits-1,blockOffBits)
@@ -292,14 +298,17 @@ class MSHR(id: Int)(implicit p: Parameters) extends L1HellaCacheModule()(p) {
   // Acquires on the same block for now.
   val cmd_requires_second_acquire = 
     req.old_meta.coh.requiresAcquireOnSecondaryMiss(req.cmd, io.req_bits.cmd)
-  val states_before_refill = Vec(s_wb_req, s_wb_resp, s_meta_clear)
+  // Track whether or not a secondary acquire will cause the coherence state
-  val sec_rdy = idx_match &&
+  // to go from clean to dirty.
-                  (states_before_refill.contains(state) ||
+  val dirties_coh = Reg(Bool())
-                    (Vec(s_refill_req, s_refill_resp).contains(state) &&
+  val states_before_refill = Seq(s_wb_req, s_wb_resp, s_meta_clear)
-                      !cmd_requires_second_acquire))
   val gnt_multi_data = io.mem_grant.bits.hasMultibeatData()
   val (refill_cnt, refill_count_done) = Counter(io.mem_grant.valid && gnt_multi_data, refillCycles)
   val refill_done = io.mem_grant.valid && (!gnt_multi_data || refill_count_done)
+  val sec_rdy = idx_match &&
+                  (stateIsOneOf(states_before_refill) ||
+                    (stateIsOneOf(s_refill_req, s_refill_resp) &&
+                      !cmd_requires_second_acquire && !refill_done))
 
   val rpq = Module(new Queue(new ReplayInternal, p(ReplayQueueDepth)))
   rpq.io.enq.valid := (io.req_pri_val && io.req_pri_rdy || io.req_sec_val && sec_rdy) && !isPrefetch(io.req_bits.cmd)
@@ -308,7 +317,7 @@ class MSHR(id: Int)(implicit p: Parameters) extends L1HellaCacheModule()(p) {
 
   val coh_on_grant = req.old_meta.coh.onGrant(
                           incoming = io.mem_grant.bits,
-                          pending = req.cmd)
+                          pending = Mux(dirties_coh, M_XWR, req.cmd))
   val coh_on_hit =  io.req_bits.old_meta.coh.onHit(io.req_bits.cmd)
 
   when (state === s_drain_rpq && !rpq.io.deq.valid) {
@@ -344,10 +353,12 @@ class MSHR(id: Int)(implicit p: Parameters) extends L1HellaCacheModule()(p) {
     when(cmd_requires_second_acquire) {
       req.cmd := io.req_bits.cmd
     }
+    dirties_coh := dirties_coh || isWrite(io.req_bits.cmd)
   }
   when (io.req_pri_val && io.req_pri_rdy) {
     val coh = io.req_bits.old_meta.coh
     req := io.req_bits
+    dirties_coh := isWrite(io.req_bits.cmd)
     when (io.req_bits.tag_match) {
       when(coh.isHit(io.req_bits.cmd)) { // set dirty bit
         state := s_meta_write_req
@@ -379,7 +390,7 @@ class MSHR(id: Int)(implicit p: Parameters) extends L1HellaCacheModule()(p) {
   val meta_hazard = Reg(init=UInt(0,2))
   when (meta_hazard =/= UInt(0)) { meta_hazard := meta_hazard + 1 }
   when (io.meta_write.fire()) { meta_hazard := 1 }
-  io.probe_rdy := !idx_match || (!states_before_refill.contains(state) && meta_hazard === 0) 
+  io.probe_rdy := !idx_match || (!stateIsOneOf(states_before_refill) && meta_hazard === 0) 
 
   io.meta_write.valid := state === s_meta_write_req || state === s_meta_clear
   io.meta_write.bits.idx := req_idx